Many power supplies have voltage and current limiters in order to suppress undesirable and spurious current and voltage spikes. Sometimes the voltage spikes are generated by electrical equipment using the same power line, other times the transient surges are caused by the power supply being quickly switched between on and off positions.
In order to avoid damage to the power supply, inrush current needs to be suppressed. In many conventional power supplies there is an input capacitor or a capacitor bank. The input capacitor or the capacitor bank charges when the power supply is initially turned on. In this state, the capacitor or the capacitor bank acts as a short-circuit and does not limit input current. As a result, the inrush current may activate a circuit breaker or even overload a circuit near the power input.
To reduce the inrush current most conventional power supplies are provided with thermistors and relays. For example, U.S. Pat. No. 5,420,780 discloses a circuit for limiting an inrush current in a DC power supply. The inrush limiting circuit is placed between an alternating current (AC) input power line and a power supply input capacitor bank. A thermistor is located between the capacitor bank and an input diode bridge to limit an initial inrush current. Once the input capacitor bank is fully charged, an insulated-gate bipolar transistor (IGBT) switches a rectified line voltage to a boost power factor correction converter circuit to block the thermistor by means of a diode. This switching keeps the thermistor cold to maintain high resistance in case of a new off/on cycle.
Thermistors, however, cannot be used with large power supplies without risking damage to the thermistor. As a result, thermistors are being replaced with a common resistor shunted by a triac or a silicon controlled rectifier (SCR). For example, U.S. Pat. No. 6,055,167 discloses a circuit for limiting an inrush current wherein the input inrush current is reduced by phasing up an input voltage in a controlled manner. The circuit uses a SCR in an input bridge of the circuit to control the input inrush current. This arrangement, however, has its drawbacks in that if the SCR fails, the power supply will ultimately be damaged.
Another problem associated with power supplies is high current peeks and very high currents in the circuit that cause damage to the power supply when short-circuiting the direct current (DC) outputs. Conventional power supplies are provided with high voltage switches to suppress the short-circuit current. These switches are often costly, however, and increase the complexity of the circuit.